It has been suggested that the genome instability driven by telomere dysfunction may promote tumorigenesis. While telomeric DNA is usually maintained by the telomerase reverse transcriptase, some tumors utilize a telomerase-independent mechanism called Alternative Lengthening of Telomeres (ALT). Our laboratory has characterized a panel of liposarcomas and found that approximately equal numbers of these tumors use telomerase as ALT for telomere maintenance, while approximately 50 percent of the tumors have no evidence of either mechanism. DNA mapping arrays, used to assay genome instability, suggest that a period of increased genome instability likely precedes activation of a telomere maintenance mechanism. I propose to define the consequences of activation of each telomere maintenance mechanism on the stability of the genome. Secondly, these analyses revealed that the rpa14 gene, which encodes a member of the RPA heterotrimer involved in both DNA replication and recombination-based repair, is amplified in ALT-positive tumors. Another complex constituent, RPA70 was found to be localized to ALT-specific PML nuclear bodies. I, therefore, will determine the contribution of RPA to ALT. [unreadable] [unreadable] [unreadable] [unreadable]